1. Field of the Invention
The invention relates to methods for depositing high quality films of complex materials on substrates at high deposition rates, and apparati for effecting such methods. Particularly, the invention relates to enhanced chemical vapor deposition from liquid sources of high quality thin films of a large variety of complex metal-oxide compounds at high deposition rates, and apparati for effecting such methods. More particularly the invention relates to apparati and methods for fabricating high quality thin films of ferroelectric layered superlattice materials.
2. Statement of the Problem
There are known methods for depositing thin films of complex compounds, such as metal oxides, ferroelectrics, super-conductors, materials with high dielectric constants, gems, etc. Such known methods include RF sputtering, chemical vapor deposition (xe2x80x9cCVDxe2x80x9d), and spin coating. RF sputtering does not provide thin films of suitably high quality for practical integrated circuit uses, and it is hard if not impossible to control the stoichiometry so as to produce materials within the strict requirements of integrated circuit uses. Spin coating avoids the above defects of sputtering, but does not have good step coverage and suitably high fabrication rates for commercial uses. Present methods of chemical vapor deposition, while having good step coverage, are simply not able to form complex materials of suitable quality for integrated circuit use.
It has been recently discovered that certain layered compounds, referred to herein as layered superlattice materials (or layered superlattice compounds), are far better suited for use in ferroelectric and high dielectric constant memories than any prior art materials. These materials are highly complex, and no method is available to reliably fabricate high quality layered superlattice compounds in commercial quantities, at high deposition rates, and with step coverage that is suitable for making state-of-the-art integrated circuits. The application of known CVD methods to complex materials, such as layered superlattice materials, results in premature decomposition of the reagents and, often, a dry dust, rather than a solid material deposited on the substrate, or result in inferior quality materials that are not suitable for use as active components in an integrated circuit.
In conventional CVD methods, one or more liquid precursors are vaporized to the gaseous state using bubblers, in which carrier gas is bubbled through the liquid precursor. This process step requires that the precursor have sufficient volatility at the bubbling temperature to enable sufficient mass transfer rate for a commercially viable process. Even under good mass transfer conditions, however, the mass transfer rate is difficult to control accurately and precisely. When a plurality of liquid precursors are gasified, any uncontrolled variations in mass transfer rate and mass transport in the process streams result in fluctuations in product stoichiometry. Further, in order to vaporize, that is, gasify, sufficient quantities of liquid precursor at a commercially viable rate it is typically necessary to heat the liquid precursor during bubbling. But, the precursors used in the prior art are typically chemically unstable at the higher temperatures necessary to achieve sufficient mass transfer of the precursor from the liquid phase to the gaseous phase. As a result, premature decomposition of the chemical compounds contained in the precursor occurs. Premature decomposition causes undesirable, uncontrolled changes in the chemical stoichiometry of the process streams and the final product, as well as uneven deposition on the substrate in the CVD reactor. Premature decomposition, therefore, results in poor electronic and ferroelectric properties. Premature decomposition also leads to rapid fouling of the CVD-apparatus, necessitating frequent shut-downs for cleaning.
Another typical process for vaporizing liquid precursors in the prior art is to create a mist of small droplets by pushing the liquid through a needle syringe. The mist is usually injected directly into a deposition reactor. The temperature of the deposition reactor must be high enough to rapidly gasify the mist droplets. Such a procedure, however, does not produce a continuous stream, nor does it result in a stream comprising liquid droplets of small mean particle size with a narrow, controllable size distribution. The gasification of larger-sized liquid droplets requires higher temperatures, which inevitably leads to premature decomposition of the precursors. When the precursors decompose in the deposition reactor away from the substrate, they form particles on the substrate instead of a continuous, uniform film of material. Also, fouling of the apparatus occurs.
Still another conventional method of vaporizing the liquid precursor in a CVD process is to use an ultrasonic mist generator to form a mist of liquid droplets, and then transport the droplets into a heated zone of the deposition reactor itself to gasify the droplets at elevated temperature. It has been found that the ultrasonic mist generators add so much energy to the liquid precursors that they become chemically unstable and prematurely decompose. Also, the sizes of liquid droplets reated by the ultrasonic mist generators vary over a wide range and, therefore, are gasified at different rates. Finally, extra high temperatures are needed in the deposition reactor to gasify the liquid droplets, and the high temperatures lead to premature decomposition.
A common feature of CVD processes and apparati in the prior art is the failure to sufficiently mix misted precursors and gas-phase reactants to ensure desired control of reaction conditions in the CVD and control over the stoichiometry and quality of the deposited thin film. Also, the liquid precursors in the prior art have low vapor pressures, and they tend to decompose at the high temperatures necessary for gasifying them.
It would be helpful to have a method, an apparatus, and liquid precursors for fabricating thin films in integrated circuits in a commercially viable manner that would allow good control of stoichiometry in the deposited thin film, avoid the problem of premature decomposition, and provide the advantages usually associated with CVD processes, such as good step coverage and uniform film quality.
3. Solution to the Problem
The invention solves the above problems by providing methods, precursors, and apparati for the chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) of thin films of metalorganic compounds, particularly precursors of layered superlattice materials, that avoid premature decomposition of the reagents, provide easily controlled composition and flow rate of a gas phase reactant stream to the CVD reactor, and result in a thin film containing small grains having mixed orientation and good electrical properties.
The invention provides at least one liquid precursor containing at least one complex metalorganic compound.
The invention provides a multi-step gasification (or vaporization) process, comprising: production of a mist of each liquid precursor by a venturi mist generator; and rapid, low-temperature gasification of the mist in a separate gasifier. Preferably, the mists of each liquid precursor are combined and mixed in a separate mist mixer before gasification.
The invention provides a method and an apparatus for forming a mist using a venturi mist generator. The mist generator produces a mist of variable, controllable mass flow rate, comprising droplets of narrow, controllable size distribution. The mist droplets have a mean droplet diameter of less than one micron, and preferably in the range of 0.2 to 0.5 micron. Because the mass flow rate and chemical composition of the mist is known, it is possible to deposit a thin film of uniform, desired composition and stoichiometry.
The invention provides for a venturi mist generator comprising a variable gas inlet passage and a variable gas passage throat. The invention also provides for a venturi mist generator comprising a variable liquid inlet passage and a variable liquid passage throat. Such variability allows more flexible control of mass flow rate and mist droplet size.
The invention provides a method and apparatus for combining and mixing the mist streams exiting venturi mist generators. In this way, the invention provides a well-mixed mist that maintains uniform, desired stoichiometry of reagents in the mixed mist stream entering the gasifier.
The invention provides for a gasifier in which the liquid reagents contained in the mist droplets in a flowing mist stream are gasified quickly at low temperature before entering a deposition reactor, thus avoiding premature decomposition at elevated temperature during gasification. This is possible because the droplets, being small, have a large surface to volume ratio and are moving at a finite velocity in the carrier gas through the gasifier. As a result, heat transfer to the liquid and mass transfer of liquid to gas are enhanced. The enhanced heat transfer rate enables the latent heat of vaporization, required to gasify the liquid droplets in the mist, to transfer to the liquid at a temperature below the ranges in which substantial premature decomposition of the reagents could occur. The enhanced mass transfer increases the rate at which a given mass of liquid molecules can move into the gas phase, thereby reducing the time period of gasification during which the precursor reagents are held at elevated temperature. It is an object of the invention to gasify liquid mist droplets at a temperature not exceeding 300xc2x0 C. Preferably the mist droplets are gasified at a temperature in the range from 100xc2x0 C. to 200xc2x0 C.
Another aspect of the inventive venturi-misting and subsequent low-temperature gasification of precursor liquid droplets is the use of low-volatility reagent compounds, which otherwise would not be usable in a CVD process because of the inability of the prior art to gasify the reagents at low temperature at a rate sufficiently fast for commercial fabrication.
The invention provides for flowing the gasified precursor through insulated tubing at ambient temperature, thus avoiding condensation by cooling, and premature decomposition of reagents by heating to elevated temperature
The invention provides for mixing gasified precursor and oxidant gas in a mixing chamber or an oxidant mixer at low temperature before they enter the interior space of the deposition reactor. This results in a well-mixed reactant gas achieved at low temperature to avoid premature decomposition of the reagents.
It is an object of the invention to cool the interior space of the deposition reactor so that the reactant gas does not prematurely decompose at unnecessarily high temperatures.
The invention is useful for fabrication of thin films of ceramics, glasseous materials, electrically-active materials, including ferroelectric materials and high dielectric constant materials from liquid sources including sol-gel or MOD formulations. Preferably the liquid sources comprise metal alkoxide compounds. The invention in particular provides a method of fabricating an integrated circuit having at least one ferroelectric layered superlattice thin film. Preferably the integrated circuit is a non-volatile memory.
The invention provides liquid precursors containing metal moieties in effective amounts for forming layered superlattice material, in particular, ferroelectric layered superlattice material.
The invention provides for CVD precursors utilizing methoxides, ethoxides, butoxides, propoxides and other alkoxide compounds with which CVD precursors may be made for almost any layered superlattice material. Preferably the invention uses a multi-metal polyalkoxide reagent. Compared with other metalorganics used in CVD, the polyalkoxides have high volatility, good chemical stability, and high decomposition temperatures.
Another aspect of the invention is the use of multi-metal polyalkoxide precursors containing a plurality of chemical constituents of the desired thin film in order to reduce the total number of liquid precursors.
The invention provides a liquid precursor comprising a metalorganic compound dissolved in an organic solvent. The invention provides for a low volume fraction of solvent in a liquid precursor solution.
Preferably there is only one liquid precursor containing all reagents necessary for forming the desired thin film. If a plurality of liquid precursors is used to fabricate a layered superlattice material, preferably there is one metalorganic precursor containing superlattice generator atoms, and one multi-metal polyalkoxide precursor containing A-site and B-site atoms. Preferably, the metalorganic precursor contains a bismuth-containing metalorganic compound that reacts with a multi-metal polyalkoxide precursor to produce a bismuth-layered superlattice material. The preferred method provides for mixing all precursors used in the CVD process in a common solvent, such as tetrahydrofuran, prior to the misting step. In an alternative method, the invention provides for forming a mist of each liquid precursor separately, and then combining the separate mist streams and mixing them before gasification (vaporization).
In a preferred embodiment, the invention provides for a lead-containing organic precursor in order to produce a Pb-containing Bi-layered superlattice compound.
The invention provides for flowing the reactant gas comprising gasified precursor and oxidant gas through a showerhead injector towards the substrate. This results in a fresh supply of reactant gas of desired composition at the surface of the heated substrate, where it reacts. This enhances formation of a continuous thin film of solid material with uniform, desired stoichiometry.
The invention provides for decomposition of vaporized reagents and formation of the integrated circuit thin film in the deposition reactor on a substrate heated to a temperature in the range of 300 xc2x0 C. and 600 xc2x0 C. The thin film comprises material in an amorphous phase, a partially crystalline phase, or a polycrystalline phase. In particular, the invention provides for formation of a thin film containing metal moieties in effective amounts for forming layered superlattice material.
The invention provides for a treating process at elevated temperature after the thin film is formed on the substrate. In the preferred embodiment of the method, the thin film is crystallized or recrystallized in an oxygen furnace anneal step, typically at a temperature of from 500xc2x0 C. to 900xc2x0 C., preferably 750xc2x0 C. In an alternative embodiment of the preferred method, a rapid thermal processing (RTP) anneal is conducted in addition to the oxygen furnace anneal. In the RTP anneal, the temperature is ramped over a range of from 1xc2x0 C. per second to 300xc2x0 C. per second and up to a temperature of from 500xc2x0 C. to 850xc2x0 C. for a holding period of from 3 seconds to 5 minutes. Preferably a combination of a furnace anneal and an RTP anneal is used. It is an object of the invention that treating the thin film of solid material results in a phase including more grains with a high polarizability orientation than prior to said step of treating.
The invention also optionally provides for an ion implantation step after the deposition of the thin film and prior to the treating process. This ion implantation step creates ion damage on the surface which provides a large number of crystallization nucleation points of different orientations.
The invention provides for depositing an electrode or electrical contact on the material, such as a layered superlattice material, followed by a second anneal, preferably a furnace anneal at from 600xc2x0 C. to 900xc2x0 C. for a period of 15 minutes or more.
Preferably each of the heating processes, that is, the deposition process at the substrate, the treating of the thin film (furnace and/or RTP anneal), and the second anneal after contact formation, takes place at the same or a higher temperature than the preceding treating step.
The invention provides for ion-coupled plasma (ICP) excitation of the reactant gas in the deposition reactor, which accelerates the rate of decomposition and reaction by overcoming kinetic barriers to reaction without adding heat to the reaction.
The invention provides for UV-irradiation of the reactant gas in the deposition reactor to enhance reagent decomposition and electronic properties of the deposited thin film.
The invention also includes a step of prebaking the substrate in an oxygen furnace at a temperature of between 500xc2x0 C. and 1000xc2x0 C. prior to performing the CVD deposition step.
The methods described above result in layered superlattice materials with good electronic properties, such as high polarizability, high dielectric constants, and low leakage currents. This is believed to be due to a crystalline orientation that results in good electronic properties.